DEBUGGER/DEBUGGER_FLAGS/DEBUGSERVER/DEBUGSERVER_FLAGS are evaluated by the
main Makefile.include or by file included by it.
Their value does not need to be exported.
Testing
-------
`git diff --word-diff` only reports `export` being removed.
`git show --stat` reports `55 insertions(+), 55 deletions(-)`
Which is the same amount as lines that where matching
`export[[:blank::]]\+VARIABLE`.
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